It is well known that the core-ends of large generators have two flux-sources (i.e., current carrying conductors)—an end-winding carrying direct currents in the rotor and an end-winding carrying alternating currents in the stator. It also has a large number of flux-receivers (i.e., conductive components that can carry induced currents)—stepped end-laminations, flanges, clamp bars, keybars, vent duct spacers, vent support sheets, retaining rings, centering disks, rotating shaft etc. The two flux sources radiate a fringe-flux field that rotates at synchronous speed. This fringe flux impinges large conductive surfaces of all flux-receivers, inducing large eddy currents in them. Further, in different abnormal conditions of operation, the fringe flux concentrates at different locations in the flux receivers wherein temperatures can be hotter than average. Ensuring that temperatures of all hot spots in all flux receivers in all abnormal conditions are within design limits (to prevent derating of the generator) is a major challenge to the core-end designer. The end laminations also carry the useless fringe-flux in addition to the usable main flux; hence, the flux density in them is so close to the saturation limit that excess flux may spill into air; this induces more eddy current that can overheat, forcing the generator to operate at lower power rating. Further, if the insulative coating on the end-lamination surfaces is defective and prone to shorts, the exciting voltage of the fringe-flux drives eddy currents through the shorts and can melt the iron; such microscopic shorts burn the insulation around the hot spot, propagating the melt zone, leading to core failure. Thus reduced power and core failure risks are the additional drivers to reduce overheating at all flux-receiver hot spots.
FIG. 1 is a longitudinal section view of the core-end 50 of a large generator showing the two flux-sources 41, 33 and multiple flux receivers 53, 18, 20, 22, 55, 40, 52. The flux-source 41 is a rotor end-winding encased between steel shaft 40, retaining ring 55 and a centering ring 52. It carries direct current, which radiates rotating flux in free space. The large tooth surfaces of end-iron 20 receive this rotating fringe flux at ˜90°; it induces large eddy currents that create a hot spot 18. A second flux-source 33 is a stator end-winding carrying alternating currents that radiates alternating flux. Near the flange bore 17 this flux is received radially at 90° inducing large eddy currents, creating another hot spot 17. Thus, typical hot spots highlighted in FIG. 1 can be the end-iron 18 (due to axial component of rotor flux source 41) and the flange bore 17 (due to radial component of stator flux source 33). The generator can be conceived as two rotating bar magnets, one in the rotor and other in the stator that chase each other. Their angular displacement defines the phase angle. Normally this angle is lagging, both magnets repel, so the stator bar magnet prevents flux from the rotating bar magnet from entering so the stator is somewhat less saturated. However, in one abnormal condition—termed leading phase angle—both magnets attract, so the stator magnet invites more flux from rotating magnet; so rotating magnet's axial component saturates the end-iron and overheats the end-laminations. In another abnormal condition—termed sudden short circuit—large currents flow through stator windings, so the stator magnet becomes so powerful at the protrusion of stator bars so its large radial component overheats the inner periphery of flange 17 and its shield. Its spatial component also hits and overheats the rotating retaining ring 55. Thus, devices to limit core-end heating should reduce not only the axial component but also the radial component of fringe flux in diverse locations under diverse abnormal conditions.
Over the past century, several devices have been developed to reduce core-end heating. Most of them attempt to reduce the flux received by the flux-receivers (instead of reducing the flux radiated by the flux-sources). Such flux receiver-focused devices can be grouped into modifiers (viz., step-iron, flux-shunts and short-rotors) and shields. The modifiers tend to alter the geometry or materials of the flux-receivers so that less flux impinges them. Step-iron and short rotors for example change the geometry of end-iron and rotor that increases the path reluctance hence reduces the flux received. Flux shunts add magnetic materials to divert part of the source flux away from flux receivers. The flux-receiver shield is a device of high electrical conductivity conforming to the shape of a flux receiver it protects and separated from it by a gap. Flux from the source hits the shield creating eddy currents whose fields repel the source flux; so less flux is received by the flux-receiver. For example, flux shield on a flange is an annular ring of copper, shaped like a flange and separated by a gap. Flux shield on end-iron is a conductive plate positioned over the end-laminations. Prior-art has developed such diverse devices in an attempt to reduce hot spot temperatures in individual core-end parts. Almost all of them accomplish this by reducing the flux received by the flux receiver without disturbing the flux sources as discussed below.
End-stepping 20 (to reduce the flux received by end-iron 18 only) is well-known in the prior-art. The end-steps 20 involve filling the last few inches of the core with stepped end-packages; teeth are possibly slit. The longer path in the stepped region increases path reluctance, reducing the flux received by the end-laminations and hence reducing the eddy heat. The funnel shape also eases the entrance losses. However, there are some unresolved issues. a) The effect is one of redistribution of axial flux, rather than its reduction, so the lower peak flux in the conical gap is accompanied by a rise elsewhere per Mecrow (1989). b) End-stepping requires higher excitation current, increasing losses in the rotor per U.S. Pat. Nos. 6,525,444 and 7,057,324. c) End-stepping reduces the clamping pressure in the bar and tooth tip, which could make them, rattle and break. d) Gradients in the axial flux induce additional eddy heat. e) Abrupt steps in the flow could result in coolant-starved areas creating new hot spots. f) More eddy current is induced inside the bar strands creating new hot spots; to counter this U.S. Pat. No. 6,455,977 disclosed an alternative profile that contains an unstepped zone facing the retaining ring. Alternatives ways to reduce overheating of the end-iron include: low-loss amorphous metal (U.S. Pat. No. 6,525,444), low-loss M4 grade steel (U.S. Pat. No. 7,265,473) or laminations with easy-axis of magnetization along teeth (U.S. Pat. No. 7,057,324). However, amorphous metals suffer from lower saturation limit and enormous cost, M4 steel does not alter the basic eddy loss mechanism and easy teeth require an expensive dies to cut laminations with easy axis along tooth.
Flux shunts on end-iron (to reduce the flux received by end-iron 18 only) are magnetic materials that divert some or all fringe-flux. However, a flux shunt attracts more flux, so they must be designed carefully. Flux shunts differ in location, geometry and magnetic materials. For example, U.S. Pat. No. 3,714,477 disclosed a flux shunt positioned about tooth, made of laminations with surfaces at 90° to fringe-flux. Such surfaces normal to the fringe flux induce large eddy currents, overheating it. U.S. Pat. No. 4,258,281 discloses an alternative flux shunt with lamination surface at 0° to fringe-flux; but such construction is structurally weak. U.S. Pat. No. 4,281,266 disclosed an alternative flux shunt made of laminations bent and bolted to a tapered finger-ring. However, since lamination surface is at 90° to the fringe-flux, large induced eddy currents overheat it. Recent U.S. Pat. No. 6,608,419 and application 20030201689 overcome these problems with an alternative flux shunt of powder-iron blocks in the conical air gap. However, powder-iron blocks saturate fast, causing them to overheat. U.S. Pat. No. 4,054,809 disclosed an alternative flux shunt between the flange and end windings, comprised of wound iron wire in an epoxy cast. However, epoxy expands faster leading to delamination. U.S. Pat. No. 1,769,816 discloses an alternative flux shunt of annular ring of magnetic material over retaining ring. However, the magnetic ting attracts more flux increasing losses. A short rotor (to reduce the flux received by end-iron 18 only) is another well-known strategy. However, shortening a rotor reduces the active length of the radial gap, reducing the power. Restoring to rated power requires an increase in the excitation current, which increases rotor losses.
Improved flanges (to reduce the flux received by flange 53 only) rely on materials with increased resistivity, such as better cast iron (U.S. Pat. No. 7,843,104), aluminum, nonmagnetic stainless steel (U.S. Pat. No. 6,455,977), or radially segmented plates that breakup eddy currents (U.S. Pat. No. 6,858,967). Fully laminated flanges in U.S. Pat. No. 4,638,199 and EP No. 0171571 are made by bonding laminations stacks that are stepped from inner to outer periphery of the core. However, since lamination surface is at 90° to the fringe-flux, they produce severe eddy heat. Flanges comprising non-magnetic stainless steel plates sandwiching a flux shunt of laminations at 90° to fringe flux are also disclosed in U.S. Pat. No. 6,858,967. However, such laminations produce large eddy heat.
Flux shield on flanges (to reduce the flux received by flange 53 only) shown in U.S. Pat. No. 3,714,477 is an annulus made of copper adjacent to the flange and spaced with a venting passageway. Analysis by Mecrow (1989) indicates that minimal losses occur at about 1.5 times skin depth. Such shields force the source flux to flow around the radial corners, thereby increasing path length and reducing the flux. However the flux and eddy heat get concentrated around the shield's inner periphery, especially in wrap-around shields. Thus, even with loss optimization the shield invariably requires better cooling means, especially in high-powered generators. Improved shield cooling means include ducts (U.S. Pat. No. 3,435,262), brazed fins (U.S. Pat. No. 4,031,422, DE 2638908), channels, grooves (U.S. Pat. Pub. No. 2009/0184594), shield/flange isolation (U.S. Pat. No. 3,886,387).
Flux shields on end-iron (to reduce the flux received by end-iron 18 only) disclosed in U.S. Pat. No. 4,152,615 is in the form of conductive plates over tooth. Alternative flux shield disclosed in U.S. Pat. No. 3,731,127 is a thin conductive shell wrapped around the protrusion of stator bars. An active shield disclosed in U.S. Pat. Pub. No. 20050121992 consists of a flux sensor, a controller to compute currents required to produce a canceling flux and a power source that applies such currents to a flux-canceling coil. However, the sensors and exciting coils are located in the straight segment of the end-windings, so the flux radiated by the bent portion could still overheat the core-end.
From this short review, it is clear that the prior-art focused on flux receivers and constructed diverse devices to reduce their overheating. However, these diverse devices require great attention to details in the design of each device, their support structure and cooling. The losses in all these devices also add up, reducing the efficiency. In contrast, the present invention focuses on the two flux sources and presents flux-shields on the flux sources. Designing two flux shields is much simpler and economical than design of a variety of shunts, shields, end-steppers and integrating their support structure into the core-end. Further, reducing the flux at the source—as presented in this invention—prevents overheating of all flux receivers under all abnormal conditions.